Depletion in fossil energy resources has been of concern in recent years, and alternative new energies have been studied. Photovoltaic generation is especially attracting attention as a promising energy resource. As a method of photovoltaic generation, a crystal solar battery using a monocrystalline silicon, a polycrystalline silicon, or the like, and a thin film solar battery using an amorphous silicon or the like have been known. Since the thin film solar battery needs less row material per unit area, its low cost has been attracting attention.
The thin film solar battery module includes, on a substrate typically made of a glass, a solar battery cell formed by a transparent electrode layer made of a transparent conductive oxide such as ITO, SnO2, or ZnO, a photoelectric conversion layer made of an amorphous silicon or the like, and a back electrode layer made of a metal such as Al, Ag, or Cr. A plurality of solar battery cells are provided on the substrate, and these solar battery cells are connected together in series or in parallel for integration.
On a back surface of the solar battery cell, a sealing layer made of a thermosetting resin such as ethylene-vinyl acetate copolymer (EVA), and a protective layer made of polyvinyl fluoride or using a composite film containing polyvinyl fluoride are laminated. The substrate and this laminate are crosslinked and cured by further heating and melting the sealing layer or sealed by using a vacuum laminating method or the like, thereby obtaining a solar battery module.
The reason why EVA is used here as the sealing layer is that the refractive index of EVA is close to that of a glass and it is inexpensive. However, weathering characteristics of EVA, such as water resistance, moisture resistance, and alkali resistance, are poor. Therefore, water has been easily entered from a side surface of the solar battery module. If water enters inside the solar battery module, not only the sealing layer made of a resin layer but also the electrical wiring, the electrode layer, and the photoelectric conversion layer are likely to corrode. That is, there has been a problem that the long-term reliability is reduced.
The cost of solar battery power generation can be regarded also as the cost/life of a solar battery. If the life of a solar battery is shortened, the cost is increased, thereby preventing the spread of solar battery power generation as an alternative energy to the fossil energy resources. Therefore, various proposals have been made regarding an improvement in the long-term reliability.
For example, Patent Literature 1 discloses a technique using a cover material in which a transparent high moisture-proof film and a transparent highly light-resistant film excellent in ultraviolet light resistance are integrally laminated.
Moreover, Patent Literature 2 discloses a technique in which a tetrafluorinated ethylene-tetrafluoroethylene copolymer, which is an ethylene-tetrafluoroethylene copolymer (ETFE) and has a surface composition by ESCA such that an amount of oxygen is in a range of 5 to 15 at % and an amount of nitrogen is in a range of 1 to 3 at %, or a tetrafluorinated ethylene-hexafluoropropylene copolymer (FEP) is used as a surface protecting material.
Moreover, Patent Literature 3 discloses a sealing structure formed by a main sealant covering a central part of a back surface of a solar battery cell, and a water vapor barrier material covering a peripheral part of the back surface. Here, the water vapor barrier material has a characteristic such that the water vapor transmission rate is 1 g/m2·day per 100 μm of film thickness.